Honeycomb structures have been used in many structural applications. Honeycombs have been widely used in both aircraft and aerospace industries to reduce weight while maintaining mechanical strength. For example, honeycombs are used in cargo floors in commercial aircraft, are used as skin panels of military aircraft wings, are used in payload shrouds on space launch vehicles and are used as optical structures in spacecraft systems. Honeycomb structures are also used as energy absorbers such as barriers in automobile crash tests. The energy absorption is based on the work done through elastic buckling, plastic yielding, and brittle crushing of the cell walls.
In analytical modeling, such as finite element analysis (FEA), honeycomb structures are generally assumed to be orthotopic. Material properties such Young's moduli, shear moduli, and Poisson's ratio, and inelastic characteristics, yield strength, and the failure strength or strain are needed as input for these models. All the mechanical property information must be accurately determined and provided as input for the computer codes before any reasonable prediction can be obtained using the codes.
At the present time, only limited material property information for honeycomb is available. Such data as is available is primarily simple compression data because compression testing is simple and easy to implement. Very little data is available for shear or tension loading, in particular, at high strain rates. When shear or tension data are available, they are limited to modulus measurements in the elastic range, or to incipient yielding. At higher tension or shear loads, the honeycomb separates at the bonding interface with the face sheet, making it impossible to determine the tension and shear properties of the honeycomb structure itself. Thus it is difficult to address the overall constituent behavior of honeycomb structures. For completely assessing the failure of a honeycomb structure, data from simple tension, simple compression, simple shear, and various combined stress fields are needed. The fact that little of the needed data is available for analytical prediction has established a high need to generate these data experimentally.
Hexcel Corporation has long provided data on honeycomb structures. An exemplar configuration of an aluminum honeycomb, designated as 1/4-5052-0.004-7.9, has a cell size of 0.25 inches, is made from 5052 aluminum, has a wall thickness of 0.004 inches and a density 7.9 lb per cubic foot. The width direction shear strength data of this Hexcel honeycomb ranges from 390 to 440 psi. The tension shear or compression shear is what has been used to determine the shear strength. This data has limitations, in that the data book indicates that the honeycomb is not being subjected to pure shear but to a combination of shear and tension/compression. The tensional compression component varies with core thickness so that thicker cores will have a lower apparent shear strength than thinner cores. This method of testing does not generate valid shear mechanical properties yet this method for testing honeycomb specimens has been used for more than 40 years. Accurate shear failure data are needed in establishing failure criteria. Currently there is no test method existing to conduct a full range of valid shear-strain tests on core material up to failure. These and other disadvantages are solved or reduced using the invention.